Description (Adapted from Application): By preparing water-soluble complexes, which differ in shape and size from the currently available gadolinium-based MRI contrast agents--the parent complex is approximately spherical with about a 12A diameter--the investigators wish to satisfy the need for new target-specific contrast agents. They will prepare water-soluble derivatives of an octanuclear Fe(III)-oxo-pyrazolato cluster, Fe8(mu4-O)4(mu-pz)12Cl4 (pz=pyrazolato anion), so that they can be screened as potential contrast enhancing agents for in vivo MRI applications. The parent cluster has a large magnetic moment, chemical stability under a wide range of conditions, and several peripheral sites available for substitution, and thus, manipulation of its size, shape, and solubility. As a reliable synthesis of this hydrophobic cluster is established, they will now prepare its water-soluble derivatives by introducing hydrophilic groups at its periphery by two methods: (1) substitution of the terminal chloride ligands, and (2) substitution at the sterically unhindered 4- position of the pyrazole rings. Once the new octanuclear clusters are fully characterized by standard methods, their relaxivity and biological activity will be determined.